1. Field of the Invention
This invention relates to specimens rotating devices, and more particularly to a specimens rotating device suitable for use with an ion implantation apparatus.
2. Description of the Prior Art
Ion implantation apparatus have in recent years been required to increase a throughput by increasing as much as possible the amount of an ionic current and reducing the time required for performing an implantation operation. An increase in the amount of ions by tenfold would increase by tenfold the heating power of the ions to which the specimens or wafers are exposed, thereby causing an abrupt rise in temperature.
To avoid a rise in temperature, it has hitherto been usual practice to arrange a plurality of wafers in a peripheral portion of a rotary plate in an ion implantation chamber and rotate the rotary plate at a velocity of rotation of about 250 revolutions per minute to lead the plurality of wafers successively to an ion implantation position to carry out ion implantation operations repeatedly. By this arrangement, the wafers are exposed to irradiation of ions in the form of pulses, with one revolution of the rotary plate serving as one cycle, so that the wafers can be naturally cooled during the periods other than the periods for the pulses.
Such natural cooling is not enough to obtain satisfactory cooling of the wafers. Thus, the present practice is to perform cooling of the central portion of the rotary plate with water thereby to cool the wafers through the rotary plate. To increase the effects achieved in cooling the wafers would require an improvement in the heat transfer performance of the rotary plate which in turn would require an increase in the thickness of the rotary plate. Thus, the rotary plate would become heavy in weight and a load applied to a motor for driving the rotary plate would rise, thereby not only making it necessary to use a complicated device for effecting control of the revolution of the rotary plate but also causing the rotary plate to become unbalanced during rotation and make a large noise.
Proposals have been made to lead water from the central portion of the rotary plate to the vicinity of the wafers arranged in the peripheral portion of the rotary plate so as to directly cool the wafers with water as much as possible. However, an ion implantation chamber is generally maintained at a vacuum of about 10.sup.-5 to 10.sup.-6 Torr and the channels through which the water flows in the rotary plate should be kept at a pressure given as the sum of the atmospheric pressure and the pressure of water at which it should be forced to flow, so that the rotary plate should be given with a thickness large enough to withstand the large pressure differential between the pressure in the water channels and the internal pressure of the ion implantation chamber. As a result, the rotary plate would have its weight increased to an extent such that the mechanism for controlling the revolution of the rotary plate would become disproportionately large in size and the rotary plate would become unbalanced during rotation, thereby making a large noise.